Well, a sideslip to landing is such an extreme case of sideslip that any non-linearity in the coupling will show up. Typically, on aerobatic planes, the mix is used primarily to help maintain a given line (i.e. vertical, 45, horizontal) when making subtle corrections with the rudder (which induces sideslips).

If you set the mixing to hold solid knife edge at a given airspeed, then it will hold that knife edge at that airspeed. No guarantees on other lines or airspeeds... that will vary from plane to plane. Some planes like one particular mix value for all times, others require more or less correction in a different scenario. For instance, on a horizontal line with wings level, the roll couple could very well be different than it is in knife edge, because you've now introduced lift on the wings and placed each wing at a different sweep angle relative to the airflow.

The set-up is really the most critical step in making an excellent competition aircraft (aside from making sure it's built straight). The best plane on the market with a poor setup will fly poorly, where a lesser airframe with a good setup can be made to fly very well. Mixing out the coupling is, of course, just one of many steps in the complete setup process, but it is a critical one. Things vary so much with different planes, I've set up planes with two different rudder to aileron mix values (on a switch) for different particular maneuvers, and other planes seem to behave perfectly with a single mix value. Either way, once they're set up, they both fly good.

One last consideration...
How long are you at knife edge in any IMAC sequence? How much time is spent correcting horizontal, vertical, and 45 lines with rudder? With that in mind, knife edge is an excellent starting point for setting rudder to elevator/aileron mixing, but it may not be the final desired configuration.

Baron